Glycoproteins are proteins that have one or more covalently attached sugar polymers. N-linked protein glycosylation is an essential and conserved process occurring in the endoplasmic reticulum of eukarotic organisms. It is important for protein folding, oligomerization, stability, quality control, sorting and transport of secretory and membrane proteins (Helenius, A., and Aebi, M. (2004). Roles of N-linked glycans in the endoplasmic reticulum. Annu. Rev. Biochem. 73, 1019-1049).
Protein glycosylation has a profound influence on the antigenicity, the stability and the half-life of a protein. In addition, glycosylation can assist the purification of proteins by chromatography, e.g. affinity chromatography with lectin ligands bound to a solid phase interacting with glycosylated moieties of the protein. It is therefore established practice to produce many glycosylated proteins recombinantly in eukaryotic cells to provide biologically and pharmaceutically useful glycosylation patterns.
It has been demonstrated that a bacterium, the food-borne pathogen Campylobacter jejuni, can also N-glycosylate its proteins (Szymanski, et al. (1999). Evidence for a system of general protein glycosylation in Campylobacter jejuni. Mol. Microbiol. 32, 1022-1030). The machinery required for glycosylation is encoded by 12 genes that are clustered in the so-called pgl locus. Disruption of N-gylcosylation affects invasion and pathogenesis of C. jejuni but is not lethal as in most eukaryotic organisms (Burda P. and M. Aebi, (1999). The dolichol pathway of N-linked glycosylation. Biochim Biophys Acta 1426(2):239-57). It is possible to reconstitute the N-glycosylation of C. jejuni proteins by recombinantly expressing the pgl locus and acceptor glycoprotein in E. coli at the same time (Wacker et al. (2002). N-linked glycosylation in Campylobacter jejuni and its functional transfer into E. coli. Science 298, 1790-1793).
Diarrheal illness is a major health problem associated with international travel in terms of frequency and economic impact. Traveller's diarrhea refers to an enteric illness acquired when a person travels from a developed to a developing country. Today, over 50 million people travel each year from developed countries to developing countries and up to 50% of these travelers report having diarrhea during the first 2 weeks of their week of their stay. There has been no significant decline in the incidence of traveller's diarrhea since the 1970s, despite efforts made by the tourism industry to improve local infrastructure.
Traveller's diarrhea is acquired through the ingestion of faecally contaminated food and less commonly water. Bacteria are the main cause of traveller diarrhea's, being responsible for up to 80% of the infections. Enterotoxigenic E. coli (ETEC) is the most frequently isolated bacterium in all parts of the world associated with traveler's diarrhea, followed by Shigella spp and C. jejuni. 
Shigellosis remains a serious and common disease. In addition to causing watery diarrhea, Shigellae are a major cause of dysentery (fever, cramps, and blood and/or mucus in the stool). Man is the only natural host for this bacterium. The estimated number of Shigella infections is over 200 million annually. About 5 million of these cases need hospitalization and a million people die. Three serogroups are mostly responsible for the disease described as bacillary dysentery: S. dysenteriae, S. flexneri and S. sonnei. 
S. dysenteriae and S. flexneri are responsible for most infections in the tropics, with case fatalities up to 20%. Shigellosis occurs both endemically and as epidemics. In many tropical countries, endemic infection is largely due to S. flexneri whereas major epidemics of S. dysenteriae have occurred in Central America, Central Africa and Southeast Asia. These epidemics are major public-health risks. Infections, primarily due to S. sonnei and less frequently S. flexneri continue to occur in industrialized countries.
Conjugate vaccines have shown promising results against Shigella infections. O-specific polysaccharides of S. dysenteriae type 1 have been used to synthesize a conjugate vaccine that has elicited an immune response in mice. Such vaccines have been synthesized chemically and conjugated to human serum albumin or has been developed where the O-polysaccharide has been purified from Shigella. The O-specific polysaccharides of S. sonnei and S. flexneri also have been conjugated chemically to P. aeruginosa exotoxin and have elicited a significant immune response in mice. Additionally, they have been shown to be immunogenic and safe in humans. However, chemical conjugation is an expensive and time-consuming process that does not always yield reliable and reproducible vaccines. This leads to good manufacturing practices (GMP) problems when seeking to develop such bioconjugate vaccines on a commercial scale.